Audio Encoding and Decoding

Abstract

A method of encoding a digital audio signal, wherein for each time segment the signal is spectrally flattened to obtain a spectrally flattened signal (r) and possibly spectral flattening parameters (LPP). The spectrally flattened signal is modelled by an excitation signal comprising a first partial excitation signal (p x ) conforming to an excitation signal generated by an RPE or CELP technique, and a second partial excitation signal (P E p ) being a set of extra pulses with arbitrary positions and amplitudes. An audio bit stream as comprising the first and second partial excitation signals is generated. The extra pulses can be added to the excitation signal at positions in time that correspond to the time of occurrence of the spike, or preferably at positions in time of an RPE time grid.

1 . A method of encoding a digital audio signal, wherein for each time segment of the signal the following steps are performed:
 spectrally flattening the signal to obtain a spectrally flattened signal (r),   modelling the spectrally flattened signal by an excitation signal comprising first and second partial excitation signals,
 the first partial excitation signal (p x ) conforming to an excitation signal generated by an RPE or CELP pulse modelling technique, 
 the second partial excitation signal (p EP ) being a set of extra pulses (P) modelling spikes (S) in the spectrally flattened signal, the extra pulses having arbitrary positions and amplitudes, and 
   generating an audio bit stream comprising the first and second partial excitation signals.   
   
   
       2 . A method according to  claim 1 , wherein the one or more extra pulses (P) are added to the excitation signal (x) at positions in time that correspond substantially to the time of occurrence of the spikes (S). 
   
   
       3 . A method according to  claim 1 , wherein the one or more extra pulses (P) are added to the excitation signal (x) at positions in time on an RPE time grid. 
   
   
       4 . A method according to  claim 1 , wherein the pulses of the first partial excitation signal (p x ) and the one or more extra pulses (P) of the second partial excitation signal (p EP ) are both at positions in time on an RPE time grid. 
   
   
       5 . A method according to  claim 3  where the positions of the extra pulses are determined as the positions of several extrema of an unquantised RPE excitation signal calculated from the residual signal. 
   
   
       6 . A method according to  claim 1  wherein the audio bit stream further comprises spectral flattening parameters (LPP). 
   
   
       7 . An audio encoder adapted to encode time segments of a digital audio signal, the encoder comprising:
 a spectral flattening unit for spectrally flattening the signal to output a spectrally flattened signal (r),   a calculating unit adapted to calculate, an excitation signal comprising first and second partial excitation signals,
 the first partial excitation signal (p x ) conforming to an excitation signal generated by an RPE or CELP technique 
 the second partial excitation signal (p EP ) being a set of extra pulses (P) modelling spikes (S) in the spectrally flattened signal, the extra pulses having arbitrary positions and amplitudes, 
   and   an audio bit stream generator for generating an audio bit stream comprising the first and second partial excitation signals.   
   
   
       8 . An audio encoder according to  claim 7 , wherein the calculating unit is adapted to add the one or more extra pulses (P) to the excitation signal (x) at positions in time that correspond to the time of occurrence of the spikes (S). 
   
   
       9 . An audio encoder according to  claim 7 , wherein the calculating unit is adapted to add the one or more extra pulses (P) to the excitation signal (x) at positions in time on an RPE time grid. 
   
   
       10 . An audio encoder according to  claim 7 , wherein the pulses of the first partial excitation signal (p x ) and the one or more extra pulses (P) of the second partial excitation signal (p EP ) are both at positions in time on an RPE time grid. 
   
   
       11 . An audio encoder according to  claim 7 , where the positions of the extra pulses are determined as the positions of several extrema of an unquantised RPE excitation signal calculated from the residual signal. 
   
   
       12 . An audio encoder according to  claim 7 , wherein the audio bit stream further comprises spectral flattening parameters (LPP). 
   
   
       13 . A method of decoding a received audio bit stream (AS), where the audio bit stream comprises, for each of a plurality of segments of an audio signal:
 a first partial excitation signal (p x ) conforming to an excitation signal generated by an RPE or CELP pulse modelling technique,   a second partial excitation signal (p EP ) being a set of extra pulses (P) modelling spikes (S) in the spectrally flattened signal, the extra pulses having arbitrary positions and amplitudes,   the method comprising means for synthesising an output signal on the basis of the combined first and second excitation signals and spectral flattening parameters (LPP).   
   
   
       14 . A method according to  claim 13 , wherein the spectral flattening parameters (LPP) are generated using a backward-adaptive linear prediction algorithm. 
   
   
       15 . A method according to  claim 13 , wherein the spectral flattening parameters (LPP) are contained in the audio bit stream. 
   
   
       16 . An audio player for receiving and decoding an audio bit stream (AS), where the audio bit stream comprises for each of a plurality of segments of an audio signal:
 a first partial excitation signal (p x ) conforming to an excitation signal generated by an RPE or CELP technique,   a second partial excitation signal (p EP ) being a set of extra pulses (P) modelling spikes (S) in the spectrally flattened signal, the extra pulses having arbitrary positions and amplitudes,   the audio player comprising means for synthesising an output signal from the combined partial excitation signals and spectral flattening parameters (LPP).   
   
   
       17 . An audio player according to  claim 16  comprising means for generating the spectral flattening parameters (LPP) using a backward-adaptive linear prediction algorithm. 
   
   
       18 . An audio player according to  claim 16  adapted to use spectral flattening parameters (LPP) received with the audio bit stream (AS). 
   
   
       19 . An audio bit stream (AS) comprising for each of a plurality of segments of an audio signal:
 a first partial excitation signal (p x ) conforming to an excitation signal generated by an RPE or CELP technique,   a second partial excitation signal (p EP ) being a set of extra pulses (P) modelling spikes (S) in the spectrally flattened signal, the extra pulses having arbitrary positions and amplitudes.   
   
   
       20 . An audio bit stream (AS) according to  claim 19  further comprising spectral flattening parameters (LPP). 
   
   
       21 . A storage medium having an audio bit stream (AS) as claimed in  claim 19  stored thereon.